Hills form through at least half a dozen fundamentally different geological processes, and no single mechanism accounts for more than a fraction of the world’s hilly terrain. Some hills are pushed up from below by tectonic compression or intruding magma. Others are left behind when erosion strips away weaker rock around a harder core. Still others are dumped in place by retreating glaciers or piled grain by grain by persistent wind. Even living organisms build hills, from termite colonies raising mounds over millennia to humans reshaping the ground on a massive scale. What all these processes share is that they create relief, meaning differences in elevation across a landscape, but they do so through strikingly different mechanisms operating on timescales from decades to hundreds of millions of years.
Tectonic Uplift, Folding, and Faulting
The most powerful hill-building engine on Earth is plate tectonics. When tectonic plates collide, the crust crumples and shortens, pushing rock upward in folds. Broad, gentle folds can produce rolling hill country stretching for hundreds of kilometers, while tighter folds create steeper ridges. Fault-block hills form when a segment of crust is lifted along a fracture; one side drops while the other rises, leaving an elevated block whose surface becomes a hill or ridge. These processes operate over millions of years, but the cumulative displacement can be enormous.
Laboratory experiments modeling the interplay between tectonic uplift and erosion show that topography tends toward a steady state: when rock is being pushed up at a constant rate, the landscape adjusts until the rate of erosion matches the rate of uplift, and the average elevation stabilizes. The mean elevation of that steady state depends on how fast the rock is rising. Even when uplift stops entirely, the surface retains some residual relief rather than eroding perfectly flat.
1Journal of Geophysical Research: Solid Earth. Laboratory experiments simulating the geomorphic response to tectonic upliftThis finding helps explain why ancient, tectonically quiet landscapes still have hills. Places like the Appalachians or the Scottish Highlands stopped experiencing major uplift long ago, yet they remain hilly because erosion alone cannot reduce them entirely to flat plains in any human-meaningful timeframe.
Geomorphologists have long debated whether landscapes under tectonic stability gradually lower their slopes over time (as the nineteenth-century geologist William Morris Davis proposed) or maintain constant slope angles as long as stream power keeps pace with uplift (the model associated with John Hack). More recent numerical modeling suggests both frameworks apply, just in different settings: slope lowering dominates where uplift has ceased, while constant-slope steady states emerge where rock is still actively rising and rivers can cut fast enough to keep up.
2Earth Surface Processes and Landforms. Long‐term landscape evolution: linking tectonics and surface processesVolcanic and Igneous Intrusions
Volcanoes are perhaps the most dramatic hill-builders. A cinder cone can grow from flat ground to a hill tens or hundreds of meters tall in a matter of weeks or months during an eruption. The classic picture of cinder cones assumed that fragments launched ballistically from the vent simply fell in an arc and piled up at the angle of repose. Research into the actual geometry and internal structure of pyroclastic cones tells a more nuanced story: the shapes and layering of real cones are better explained by clasts falling from a sustained eruption column rather than following individual ballistic arcs.
3Journal of Volcanology and Geothermal Research. Controls on the growth and geometry of pyroclastic constructsNot all volcanic hills involve lava reaching the surface. A laccolith forms when magma rises through the crust, spreads sideways as a flat sheet between rock layers, and then inflates, pushing the overlying rock upward into a dome. G.K. Gilbert first described these structures during his 1875 exploration of the Henry Mountains in Utah, coining the term “laccolite” for chambers of molten rock that lifted and bent the strata above them.
4IUGS-Geoheritage. Oligocene Laccoliths and Sedimentary Rock Domes of the Henry MountainsThe Henry Mountains remain textbook examples: broad, rounded hills whose shape reflects the geometry of the solidified magma body beneath.
Laccoliths are not always gentle, slow-growing features, either. Evidence from at least one volcanic system shows that laccolith intrusion can be driven rapidly by explosive eruption, with the magma supply and the eruption acting in a feedback loop that inflates a shallow magma body while the surface bulges upward.
5PubMed Central. Rapid laccolith intrusion driven by explosive volcanic eruptionHills created this way can appear over timescales of days to years, not just geological epochs.
Erosion and Differential Weathering
Erosion destroys hills, but it also creates them. The key concept is differential weathering: when a landscape is composed of rocks with different hardness or resistance to erosion, the softer material wears away faster, leaving the harder rock standing above the surrounding terrain. A hill formed this way was not pushed up; it was left behind.
The mesas and buttes of the central Colorado Piedmont illustrate this process clearly. Their lower sections are made of poorly cemented sandstones that erode relatively easily. Their flat tops are capped by much more resistant formations, including conglomerates and volcanic tuff. What makes the story especially interesting is that those resistant cap-rock layers were originally deposited in topographic lows, essentially in valleys. Because the cap rock resisted erosion while everything around it wore down, the former valleys now stand as the highest points in the landscape. Geologists call this relief inversion: the highs and lows have swapped places over geological time.
6Geological Society of America. From buttes to bowls: Repeated relief inversion in the landscape of the Colorado PiedmontIn softer terrain the process is subtler but still active. In the loess hill country of central Saxony, Germany, the landscape is a series of gently rolling hills separated by shallow, well-defined valleys. Detailed measurements show that over the course of the Holocene (roughly the last 11,700 years), different valleys in this region have experienced very different amounts of soil loss, with one site showing roughly a meter of profile stripped away and another about two-thirds of a meter, despite similar slope gradients.
7Elsevier / Catena. Holocene sediment fluxes in a fragile loess landscape (Saxony, Germany)Variations in vegetation cover, land use, and drainage patterns determine which parts erode faster and which persist as hills, meaning that even in terrain made of a single type of sediment, erosion can selectively carve relief.
The shape of a hillslope itself reflects the balance between the different forces wearing it down. Surface water flow, soil creep, rockfall, and landslides each dominate on different parts of a slope and produce distinct slope profiles. The overall form of a hill depends on the strength of the bedrock, the available relief, and which erosional process dominates on each section of the hillside.
Glacial Deposition
Ice sheets and glaciers are prolific hill-builders, not through uplift but through deposition. As glaciers advance and retreat, they carry enormous volumes of sediment and dump it in characteristic landforms. The most studied glacial hills are drumlins: elongated, teardrop-shaped mounds that form beneath moving ice, aligned with the direction of ice flow. Drumlin fields can contain thousands of individual hills spread across landscapes that were once under ice, as in parts of Ireland, northern England, and the Great Lakes region of North America.
The internal structure of drumlins turns out to be surprisingly varied. Field studies of dozens of drumlins show a structural continuum. Some are primarily depositional, built from sediment that flowed into low-pressure zones behind obstacles on the glacier bed. Others are deformational, shaped by the ice squeezing and shearing the sediment beneath it in complex ways. Still others are erosional, carved from pre-existing layered sediments that the ice truncated and streamlined.
8Quaternary Science Reviews. The relationship between drumlins and other forms of subglacial glaciotectonic deformationThis means that even among hills that look nearly identical on the surface, the internal history can be very different.
Theoretical modeling of what happens beneath an ice sheet has shown that the same basic physics, involving the coupled flow of ice, water, and sediment at the glacier bed, can produce several different landform types depending largely on the grain size of the sediment involved. Coarser sediment tends to produce ribbed moraine (ridges running perpendicular to ice flow), while finer sediment favors elongated drumlins or even very long streamlined ridges.
9PubMed Central. An instability theory for the formation of ribbed moraine, drumlins and mega-scale glacial lineationsIce margins also leave behind hills formed in contact with stagnant, melting ice. Kame complexes, for instance, include flat-topped hills that began as lakes trapped within hollows in the decaying ice. Sediment accumulated on the lake bed, and when the surrounding ice finally melted, the lake-bed deposits were left standing as elevated platforms. A detailed investigation of the Brampton Kame Belt in northern England found that these flat-topped hills (called ice-walled lake plains) formed through the evolution and eventual collapse of drainage channels into lakes as the ice sheet broke down.
10Wiley Online Library. Complex kame belt morphology, stratigraphy and architectureWind-Built Hills
In arid and semi-arid environments, wind moves sand and silt that can accumulate into hills ranging from small coppice dunes to massive sand seas. Sand dunes are the most familiar wind-built landforms, shaped when airborne grains lose momentum on the downwind side of an obstacle and pile up. The process is ongoing and, in geological terms, fast.
Coppice dunes (also called nebkhas) form around desert shrubs that trap windblown sediment. Research in the Negev Desert tracked the rate at which sand accumulated beneath shrub canopies and found that deposition depended heavily on wind power: when wind speeds exceeded a threshold of about 10 meters per second, substantially more material was transported and trapped. At the measured rates, a coppice dune roughly 60 centimeters tall could form in about 150 to 160 years.
11Earth Surface Processes and Landforms. Factors controlling the formation of coppice dunes (nebkhas) in the Negev DesertThat is fast enough for individual shrubs to build noticeable mounds within a human lifetime, and over centuries, clusters of nebkhas can create undulating terrain that qualifies as hilly.
Larger dune systems are shaped by the same fundamental physics at grander scales. Star dunes, barchan dunes, and linear dune ridges can reach heights of over a hundred meters, dwarfing many tectonically formed hills. Their shapes depend on wind direction, sand supply, and vegetation cover. Unlike most other types of hills, dunes are mobile: they migrate downwind unless anchored by vegetation or moisture, which means wind-built hills are among the most transient landforms on the planet.
Biological and Human-Made Hills
Some hills owe their existence to living things. In the semi-arid western Cape of South Africa, evenly spaced mounds called heuweltjies dot the landscape in strikingly regular patterns. These mounds, typically a few meters tall and tens of meters across, were long debated, but research has linked them to the southern harvester termite. Termite colonies alter the soil around them, enriching it with calcium carbonate, nitrogen, and phosphorus. The colonies are highly overdispersed, meaning they space themselves out evenly, and their sustained activity over thousands of years builds and maintains the mounds. The uniform spacing of the heuweltjies directly reflects the territorial spacing of the termite colonies beneath them.
12Journal of Arid Environments. Landscape patterning created by the southern harvester termite, Microhodotermes viatorHumans are an even more prolific hill-building species. The deliberate movement of excavated earth, construction waste, and manufactured materials has created a novel category of sedimentary environment with no real pre-human analogue.
13Geological Society, London, Special Publications. An assessment of lithostratigraphy for anthropogenic depositsIn Great Britain alone, over the past two centuries, people have excavated, moved, and built up the equivalent of at least six times the volume of Ben Nevis, the country’s tallest mountain. Humans sculpt and transform the landscape through the physical modification of the shape and properties of the ground on a scale that rivals many natural geological processes.
14PubMed. Humans as major geological and geomorphological agents in the Anthropocene: the significance of artificial ground in Great BritainSpoil heaps from mining, landfill mounds, earthen fortifications, and ancient burial mounds (tumuli) are all human-made hills. Some are enormous: coal spoil tips in parts of Wales and northern England can be over a hundred meters high. Archaeologically, tells in the Middle East are mounds built up over millennia from the accumulated debris of successive settlements, each new village constructed on the ruins of the last. These artificial hills blur the line between geology and culture, and in some landscapes they are the dominant topographic feature.
Impact Craters and Hills Beyond Earth
A less intuitive hill-forming mechanism is extraterrestrial impact. When a large asteroid or comet strikes a planetary surface, the initial explosion excavates a crater, but in large craters the floor rebounds upward to form a central peak or ring of peaks. These central peaks are formed from the dynamic uplift of deeply buried rocks during crater formation, and in the largest craters they can rise kilometers above the surrounding crater floor.
15PubMed. The formation of peak rings in large impact cratersOn Earth, erosion has degraded most impact structures to the point where central peaks are hard to recognize, but on the Moon and Mercury they are preserved clearly.
Comparing Earth’s topography to that of Mars and Titan reveals just how central plate tectonics is to the specific character of Earth’s hills and ridges. An analysis of drainage patterns on all three bodies found that on Mars and Titan, large drainage systems closely follow the long-wavelength topography, meaning rivers flow in the directions you would predict from the broadest-scale shape of the surface. On Earth, they do not: rivers frequently cut across the grain of the large-scale topography. The explanation is that plate tectonics on Earth constantly creates short-wavelength relief (folds, faults, volcanic edifices) that disrupts and redirects drainage. Mars, which is tectonically dead, and Titan, which appears to generate relief at very long wavelengths, lack this short-wavelength roughness.
16PubMed. Global drainage patterns and the origins of topographic relief on Earth, Mars, and TitanIn practical terms, this means that the hilly, chaotic topography that feels so natural on Earth is actually unusual in the solar system. Most rocky and icy bodies have smoother, more predictable relief because they lack the restless crustal recycling that gives Earth its patchwork of ridges, valleys, and hills.
How Slope and Aspect Shape What Lives on a Hill
Once a hill exists, its shape profoundly influences the local environment in ways that go well beyond topography. The slope and compass direction (aspect) of a hillside control how much solar radiation it intercepts, and solar radiation dominates the surface energy balance. South-facing slopes in the Northern Hemisphere receive more direct sunlight than north-facing ones, making them warmer and drier. This drives measurable differences in near-surface temperature, evaporation rates, and soil moisture, which in turn determine which plant species can grow where.
17Ecological Modelling. Slope, aspect and climate: Spatially explicit and implicit models of topographic microclimate in chalk grasslandYou can sometimes see this with the naked eye: on a single hill, one face may support dense grass or woodland while the opposite face is bare or scrubby. In chalk grassland, for example, south-facing slopes tend to host drought-tolerant species, while the cooler, moister north-facing slopes support a different community entirely. The same principle scales up: across an entire hilly landscape, the orientation of each slope creates a mosaic of microclimates that supports far greater biodiversity than a flat plain of equivalent area would.
This ecological effect feeds back into the hill’s own evolution. Vegetation stabilizes soil and slows erosion, so well-vegetated slopes erode more slowly than bare ones. Aspect-driven differences in plant cover can therefore cause one side of a hill to wear down faster than the other, gradually changing the hill’s profile over centuries. The interaction between a hill’s shape and the life it supports is a two-way street, each constantly reshaping the other.
Why There Is No Clean Line Between a Hill and a Mountain
A question that naturally follows from understanding how hills form is: at what point does a hill become a mountain? The honest answer is that there is no universally agreed boundary. Various countries and mapping agencies have used thresholds ranging from about 300 meters of elevation (or local relief) to 600 meters or more, but these are conventions, not geological distinctions. The processes that build a 400-meter hill are the same ones that build a 4,000-meter mountain; the difference is the intensity and duration of the driving forces, combined with the rate at which erosion fights back.
This ambiguity matters practically in land-use planning, ecological surveys, and even tourism: whether a landscape is classified as “hilly” or “mountainous” can affect building codes, conservation designations, and infrastructure investment. But geologically, the distinction is arbitrary. A hill is simply relief that has not been pushed high enough, or has been worn down far enough, to earn the label “mountain” in whatever local naming convention applies. The same fold, the same laccolith, the same drumlin field lies on a continuum of scale, and the processes described above operate across that entire continuum without respecting the boundary that human language tries to draw.